This invention relates generally to numerically controlled machines and more specifically relates to precision positioning systems for numerically controlled machines.
In a conventional high-precision numerically controlled machine, a tool is moved only when a laser interferometer or other position feedback system indicates that an error exists between the actual location of the tool and a desired location as designated by a part program. This method of transferring a tool from one location to another is known as "error positioning" because an error must exist before the tool can be moved. This scheme has a major disadvantage in that there is an undesirably large lag time between the time a part program indicates that a tool should be in a new location and the time that the tool is actually moved to that location. This disadvantage reduces the control capabilities of an NC system in terms of high speed feedback precision.
In the art of diamond machining of optical components such as large metal reflective mirrors which require a contour accuracy of 10.0 microinches, it is not considered to be a cost effective approach to make the mechanical positioning components in the various axis drive means to obtain ultra precise positioning of the diamond cutting tool for this degree of contour accuracy. If "stepping motor-like" drives were utilized and the reduction unit, the lead screw, and the nut for each axis were dimensionally perfect, the tool would be positioned exactly as commanded except for the very small lag introduced by the finite compliance of the drive shafting, gearing and the finite mass of the slide. Since this degree of ultra-high precision machining is impossible from a cost standpoint and from a practical standpoint as well, due to wear of the various machine parts over the life of a cutting machine, laser interferometer feedback is used to maintain cutting tool position accuracy.